[20.08] Collisional and Rotational Evolution of a Ring-Moonlet System

K. Ohtsuki (U. Colorado)

Planetary rings such as Saturn's rings are composed of a
number of particles and moonlets, and these small bodies
undergo mutual collisions. Although physical properties of
these small bodies are poorly understood, they might be
inferred from their dynamical behavior such as random
velocity evolution, structure formation, and rotation rates.
In many of theoretical studies on planetary rings, it is
often assumed that ring particles are smooth spheres.
However, it is not realistic, and particles should have
rough surface, which should cause rotation of particles as a
result of oblique impacts. We examine the rotation of a
moonlet embedded in planetary rings caused by impacts of
ring particles, based on the results of analytic calculation
and numerical orbital integration for the three-body
problem. Taking into account the Rayleigh distribution of
orbital eccentricities and inclinations of particles, we
evaluate both systematic and random components of rotation,
the former one arising from an average of a number of
impacts and the latter one being contribution from large
impacts. Calculations for parameter values corresponding to
Saturn's C ring show that a moonlet would spin slowly in the
prograde direction if most of impactors are small particles
whose velocity dispersion is much smaller than the moonlet's
escape velocity. However, we also find that the effect of
the random component can be significant, if the velocity
dispersion of particles is as large as the escape velocity
and large impacts are common; in this case, slow rotation in
both prograde and retrograde directions would be expected.
We will also discuss other related issues, such as rotation
and accretion of ring particles.